Method for producing a multi layer fiber web from cellulose fibers

ABSTRACT

A process for the production of a multilayer fiber web from cellulose fibers by separately feeding at least two different fiber suspensions and water into a multilayer headbox, in which they are separated by separating elements from one another and from water and, after leaving the nozzle mouth of the headbox, reach an apparatus on which a web is formed, the water being transported in such a way that, after emerging from the nozzle mouth, it reaches the web-forming apparatus in the form of a layer between two layers of fiber suspensions and thus counteracts mixing of the different fiber suspensions, wherein, for improving the retention, drainage and formation, at least one retention aid and/or at least one drainage aid are metered into the fiber suspensions and/or into the water fed in.

The invention relates to a process for the production of a multilayer fiber web from cellulose fibers by separately feeding in at least two different fiber suspensions and water into a multilayer headbox, in which they are separated by separating elements from one another and from water and, after leaving the nozzle mouth of the headbox, reach an apparatus on which a web is formed, the water being transported in such a way that, after emerging from the nozzle mouth, it reaches the web-forming apparatus in the form of a layer between two layers of fiber suspensions and thus counteracts mixing of the different fiber suspensions.

In order to produce multilayer papers in a paper machine, it must be equipped with a multilayer headbox whose outlet nozzle chambers are divided into separate flow channels and which extend over the total machine width. Thus, it is possible to produce, for example, a paper consisting of three layers, passing three paper stocks having different compositions separately through an outlet nozzle chamber having three flow channels separated from one another and draining said paper stocks on a wire of a paper machine, cf. EP-A-0 939 842 and DE-A-10 2004 051 255.

DE-A-101 26 346 discloses a process and a stock feed system for loading a multilayer headbox of a paper machine, white water being fed with the aid of a single pump and the white water stream being divided into a plurality of white water part-streams into which high-viscosity stock is then metered and the part-streams thus obtained are fed via distributors to the multilayer headbox.

WO-A-03/048452 discloses a process for the production of a multilayer fiber web from at least two different fiber suspensions. The fiber suspensions and water are fed in each case separately to a multilayer headbox, in which they are separated from one another by separating elements and, after leaving the nozzle mouth of the headbox, reach an apparatus on which a web is formed, the water being transported in such a way that, after emerging from the nozzle mouth, it reaches the web-forming apparatus in the form of a layer between two layers of fiber suspensions and prevents mixing of the different fiber suspensions. The separating elements in the multilayer headbox are designed so that they are movable in the vertical direction and thus permit a change in the pressure between the fiber suspensions in the headbox.

In the literature references cited above, however, details of the fiber suspensions are not disclosed. With the use of a multilayer headbox for the production of multilayer papers, there is always the danger that mixing of the individual fiber streams occurs between the end of the headbox nozzle and the drainage part of the paper machine.

In the production of paper which consists only of a single layer, it is known that, depending on paper type, different process chemicals, such as sizes, drainage aids, flocculants, retention aids and dry and wet strength agents or mixtures of said process chemicals and biocides and/or dyes and colored pigments are used, cf. Ullmann's Encyclopedia of Industrial Chemistry, Sixth, Completely Revised Edition, Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim 2003, volume 25, pages 1 to 157, and optical brighteners, compositions for increasing the volume of the paper (so-called bulk promoters, cf. U.S. Pat. No. 6,273,995) and modified fibers, as described, for example, in WO-A-2006/048280.

Not only the composition of the process chemicals but also the time of metering of such products to a paper stock constitute an important feature in papermaking. The method and time of metering of retention and drainage aids to a paper stock has, for example, an effect on the retention, the drainage rate and the formation. Thus, for example, EP-A-0 235 893, EP-A-0 335 575, EP-A-0 310 959, U.S. Pat. No. 4,388,150 and WO-A-94/05595 disclose a process for the production of paper using a microparticle system, a cationic polymer first being metered into the paper stock, the mixture then being subjected to the action of a shear field, bentonite or silica then being added and the pulp thus obtainable being drained without further action of shear forces with sheet formation.

According to the process disclosed in DE-A-102 36 252 for the production of paper, a microparticle system comprising a cationic polymer and a finely divided inorganic component is metered into the paper stock after the last shearing stage before the headbox and the paper stock is then drained.

EP-A-0462 365 discloses organic microparticles which may be uncrosslinked or crosslinked and which in each case comprise at least 1% by weight, but in general at least 5% by weight, of ionic comonomers incorporated in the form of polymerized units. The particle size of the uncrosslinked, water-insoluble microparticles is below 60 mm, while it is less than 750 nm for the crosslinked microparticles. The organic microparticles are used in papermaking together with a high molecular weight of ionic polymer as a retention aid system which, if appropriate, may additionally comprise inorganic microparticles, such as bentonite or silica.

Furthermore, DE-A-10 2004 063 005 discloses a process for the production of paper, board and cardboard, a microparticle system comprising a cationic polymeric retention aid having a molar mass of at least 2 million and a finely divided inorganic component being used. The polymeric retention aid is metered into the paper stock at least two points and the finely divided inorganic component is metered before or after addition of the polymeric retention aid, the paper stock being subjected to at least one shearing stage either before or after the addition of the retention aid. Further microparticle systems which are used as retention aids in papermaking are disclosed, for example, in U.S. Pat. No. 6,103,065 and WO-A-2004/015200.

DE-A-10 2004 063 000 discloses a process for the engine sizing of paper, board and cardboard, where, by successive continuous addition of an aqueous dispersion of at least one reactive size and at least one retention aid to a paper stock stream having laminar flow and a consistency of not more than 2% by weight, based on dry fibers, and drainage of the paper stock with sheet formation, a procedure is adopted in which the reactive size and the retention aid are metered in with turbulent flow into the paper stock stream at a point which is after the last shearing stage and before the beginning of the drainage process.

It is the object of the invention to provide a process for the production of multilayer papers having improved formation, the mixing of the individual fiber streams during the web-forming process not taking place to the same degree as in known processes.

The object is achieved, according to the invention, by a process for the production of a multilayer fiber web from cellulose fibers by separately feeding in each case at least two different fiber suspensions and water into a multilayer headbox, in which they are separated by separating elements from one another and from water fed in and, after leaving the nozzle mouth of the headbox, reach a drainage apparatus on which a multilayer fiber web is formed, the water being transported in such a way that, after emerging from the nozzle mouth, it reaches the web-forming apparatus in the form of a layer between two layers of fiber suspensions and thus counteracts mixing of the different fiber suspensions, if, for improving the retention, drainage and formation, at least one retention aid and/or at least one drainage aid are metered into the fiber suspensions and/or into the water fed in.

According to the invention, a retention aid or a drainage aid or both products is or are metered into fiber suspensions. The addition of these aids can be effected in the papermaking process before shearing of the paper stock, between two shearing stages or after the last shearing of the paper stock. Preferably, an aqueous solution of at least one retention aid and/or at least one drainage aid is metered into the paper stock stream at a point which is located after the last shearing stage of the paper stock and before the nozzle mouth of the headbox. The addition of retention aid and/or drainage aid to the paper stock is particularly advantageously effected with turbulent flow of the aqueous formulations of the process chemicals. As a result, the distribution of these products in the paper stock which is as uniform as possible is achieved. In order to produce turbulent flow, it is possible to use, for example, the apparatus which is described in U.S. Pat. No. 6,659,636. It consists, for example, of a binary or multi-material nozzle through which water recycled from the paper machine and retention aid and/or drainage aid are passed in to a paper stock stream having laminar flow. The flow velocity of the paper stock stream is, for example, at least 2 m/sec and is in general in the range from 3 to 7 m/sec in the customary paper machines.

A process for the production of a multilayer fiber web from cellulose fibers by separately feeding in each case at least two different fiber suspensions and water into a multilayer headbox, in which they are separated by separating elements from one another and from water fed in and, after leaving the nozzle mouth of the headbox, reach a drainage apparatus on which a multilayer fiber web is formed, the water being transported so that, after emerging from the nozzle mouth, it reaches the web-forming apparatus in the form of a layer between two layers of fiber suspensions and thus counteracts mixing of the different fiber suspensions, is disclosed in WO-A-03/048452 discussed in connection with the prior art. For details, reference is made to this publication, in particular page 2, line 2 to page 5, line 32, and the patent claims. As described therein, the stream of water fed in is transported by so-called blade means inside the headbox and enters the drainage part of the paper machine between two streams of fiber suspensions. The velocity of the water stream is adapted to the velocity of the fiber streams and is, for example, in the range from 2 to 10 m/sec, preferably from 3 to 8 m/sec. The velocity of the water stream which separates the fiber streams is preferably from 1 to 25%, in general from 5 to 10%, higher than the velocity of the individual fiber streams.

At least two fiber suspensions which are processed to give a multilayer fiber web have a different composition. Thus, for example, the middle layer of a three-layer fiber web may consist of a cheap fiber, such as wastepaper, and top and bottom of the fiber web are formed from a high-quality fiber, for example bleached pine sulfate. Different types of multilayer paper webs can be produced through the choice of fibers and the additional use of further process chemicals, such as engine sizes and/or strength agents.

With the use of different fiber suspensions which differ, for example, only in the type of suspended fibers or the concentration thereof, it is possible, according to the invention, to adopt a procedure in which the same retention aid is added to all streams of fiber suspensions and it is metered in such a way that at least two streams of fiber suspensions comprise a different concentration of this retention aid. A further variant of the process according to the invention consists in the metering of at least two retention aids differing from one another into at least two streams of fiber suspensions. These fiber suspensions may be composed of the same or different fibers. In addition, it is possible for at least two streams of fiber suspensions to comprise a different concentration of retention aid.

In another embodiment of the process according to the invention, an aqueous solution of a retention aid and an aqueous dispersion of at least one filler are metered separately from one another or as a mixture into the paper stock.

However, retention aid and drainage aid can also be metered into at least one stream of the water fed in. Thus, for example, it is possible to meter an aqueous solution of a retention aid and an aqueous dispersion of at least one filler separately from one another or as a mixture into at least one stream of the water fed in. However, it is also possible to adopt a procedure in which an aqueous dispersion of a filler is metered into at least one stream of the fiber suspension and in which the water fed in comprises at least one retention aid. The retention aid fed in with the water may also be taken as an aqueous solution from a storage vessel or metered separately into the water fed in.

Further process variants consist in using at least one retention aid in combination with at least one drainage aid or in using at least one retention aid in combination with a fixing agent. Fixing agents are used in particular when the paper stock has a high cationic demand, for example a COD value of from 300 to 30 000, in general from 1000 to 20 000, mg of oxygen/kg of the aqueous phase of the paper stock.

Examples of fixing agents are condensates of dicyandiamide and formaldehyde, condensates of dimethylamine and epichlorohydrin, cationic polyacrylamides having molar masses M_(w) of from 1000 to 20 000, or hydrolyzed homo- and copolymers of N-vinylformamide having a K value of from 30 to 150, preferably from 60 to 90 (determined according to H. Fikentscher, Cellulose-Chemie, volume 13, 48-64 and 71-74 (1932) in 5% strength by weight aqueous sodium chloride solution at a temperature of 25° C. and a polymer concentration of 0.5% by weight). Fixing agents are used, for example, in an amount of from 0.02 to 2% by weight, preferably from 0.05 to 0.5% by weight, based on dry paper stock.

All retention aids which are known for this purpose from papermaking practice or from the literature can be used for the process according to the invention. They are used, for example, in an amount of from 0.01 to 0.3, preferably from 0.01 to 0.05, % by weight, based on dry paper stock. The retention aid can be selected, for example, from the group consisting of cationic, anionic, nonionic and amphoteric polymeric organic compounds or a microparticle system. The most commonly used retention aids belong, for example, to the group consisting of polyacrylamides, polymethacrylamides, polymers comprising vinylamine units and/or the microparticle systems.

Polyacrylamides and polymethyacrylamides may be nonionic, cationic, anionic or amphoteric. Polymers suitable as retention aids have an average molar mass M_(w) of at least 1 million, preferably at least 2 million and in particular at least 5 million. Nonionic polyacrylamides or polymethacrylamides are prepared, for example, by polymerization of N-vinylformamide, acrylamide and/or methacrylamide.

Cationic polyacrylamides are, for example, copolymers which are obtainable by copolymerization of acrylamide and at least one di-C₁- to C₂-alkylamino-C₂- to C₄-alkyl(meth)acrylate or a basic acrylamide in the form of the free bases, the salts with organic or inorganic acids or the compounds quaternized with alkyl halides or with dimethyl sulfate. Examples of such compounds are dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, dimethylaminoethyl acrylate, diethylaminoethyl acrylate, dimethylaminopropyl methacrylate, dimethylaminopropyl acrylate, diethylaminopropyl methacrylate, diethylaminopropyl acrylate and/or dimethylaminoethylacrylamide, dimethylaminoethylmethacrylamide, dimethylaminopropylacrylamide, dimethylaminopropylmethacrylamide and/or diallyldimethylammonium chloride. Said comonomers may also be copolymerized with methacrylamide to give cationic polymethacrylamides which comprise, for example, from 5 to 40 mol % of at least one cationic monomer, such as dimethylaminoethyl acrylate or diallyldimethylammonium chloride incorporated in the form of polymerized units. Cationic polymethacrylamides can be prepared by copolymerization of methacrylamide with at least one of the cationic monomers described above. Further cationic polymers which are used as retention aids are copolymers of N-vinylformamide and at least one of the cationic monomers mentioned above.

Anionic polyacrylamides are obtainable, for example, by copolymerizing acrylamide with at least one ethylenically unsaturated C₃- to C₅-carboxylic acid, in particular acrylic acid or methacrylic acid, and/or a monomer comprising sulfo groups, such as vinylsulfonic acid or styrenesulfonic acid, and/or a salt of said monomer. Preferred salts are the alkali metal salts, in particular the sodium salts, and ammonium salts. Anionic polymethacrylamides are prepared analogously thereto by polymerization of methacrylamide with the abovementioned monomers having acid groups. The anionic polymers comprise, for example, from 1 to 50, preferably from 5 to 40, mol % of at least one anionic monomer incorporated in the form of polymerized units.

The anionic polymeric retention aids also include copolymers which are obtainable by copolymerization of

at least one N-vinylcarboxamide of the formula

where R¹ and R² are H or C₁- to C₆-alkyl, at least one monoethylenically unsaturated monomer comprising acid groups and/or the alkali metal, alkaline earth metal or ammonium salts thereof and, if appropriate, other monoethylenically unsaturated monomers and, if appropriate, compounds which have at least two ethylenically unsaturated double bonds in the molecule.

A preferably used polymeric anionic compound of this group is a copolymer which is obtainable by copolymerization of N-vinylformamide, acrylic acid, methacrylic acid and/or the alkali metal or ammonium salts thereof and, if appropriate, other monoethylenically unsaturated monomers,

the polymeric anionic compound comprising, for example,

-   (a) from 10 to 95 mol % of units of the formula I, -   (b) from 5 to 90 mol % of units of a monoethylenically unsaturated     carboxylic acid having 3 to 8 carbon atoms in the molecule and/or     the alkali metal, alkaline earth metal or ammonium salts thereof and -   (c) from 0 to 30 mol % of units of at least one other     monoethylenically unsaturated monomer     incorporated in the form of polymerized units.

The compounds of this group can be modified so that they additionally comprise at least one compound (d) having at least two ethylenically unsaturated double bonds in the molecule, incorporated in the form of polymerized units. If the monomers (a) and (b) or (a), (b) and (c) are copolymerized in the presence of such a compound (d), branched copolymers are obtained. The ratios and reaction conditions should be chosen so that polymers which are still water-soluble are obtained. In certain circumstances, it may be necessary for this purpose to use polymerization regulators. All known regulators, such as, for example, thiols, secondary alcohols, sulfites, phosphites, hypophosphites, thio acids, aldehydes, etc., can be used (further information can be found, for example, in EP-A-0 438 744, page 5, lines 7-12). The branched copolymers comprise, for example,

-   (a) from 10 to 95 mol % of units of the formula I, -   (b) from 5 to 90 mol % of units of a monoethylenically unsaturated     monomer comprising acid groups and/or the alkali metal, alkaline     earth metal or ammonium salts thereof, -   (c) from 0 to 30 mol % of units of at least one other     monoethylenically unsaturated monomer and -   (d) from 0 to 2 mol %, preferably from 0.001 to 1 mol %, of at least     one compound having at least two ethylenically unsaturated double     bonds     incorporated in the form of polymerized units.

Amphoteric polyacrylamides and amphoteric polymethacrylamides each comprise units of cationic and of anionic monomers incorporated in the form of polymerized units. An example of this is a copolymer of acrylamide, dimethylaminoethyl acrylate hydrochloride and acrylic acid.

Polymers comprising vinylamine units are obtainable by hydrolysis of polymers comprising vinylformamide units. Polyvinylamines are prepared, for example, by hydrolysis of homopolymers of N-vinylformamide, the degree of hydrolysis being, for example, up to 100%, in general from 70 to 95%. High molecular weight copolymers of N-vinylformamide with other ethylenically unsaturated monomers, such as vinyl acetate, vinyl propionate, methyl acrylate, methyl methacrylate, acrylamide, acrylonitrile and/or methacrylonitrile, can also be hydrolyzed to give polymers comprising vinylamine units and can be used according to the invention as retention aids. The polymers comprising vinylamine units are cationic. In the hydrolysis of polymers of N-vinylformamide with acids, the salts of the polymers (ammonium salts) form, while polymers carrying amino groups form in the hydrolysis with bases, such as sodium hydroxide solution or potassium hydroxide solution. The preparation of homo- and copolymers of N-vinylformamide and the preparation of the polymers having amino or ammonium groups and obtainable therefrom by hydrolysis are known. It is described in detail, for example, in U.S. Pat. No. 6,132,558, column 2, line 36 to column 5, line 25. The statements made there are hereby incorporated by reference. Polymers comprising vinylamine units are preferably used as retention aids in the process according to the invention.

Further cationic polymeric retention aids are polydiallyldimethylammonium chlorides (polyDADMAC) and branched polyacrylamides, which can be prepared, for example, by copolymerization of acrylamide or methacrylamide with at least one cationic monomer in the presence of small amounts of crosslinking agents. Such polymers are described, for example, in U.S. Pat. No. 5,393,381, WO-A-99/66130 and WO-A-99/63159.

Other suitable cationic retention aids are polyamines having a molar mass of more than 50 000, modified polyamines which are grafted with ethylenimine and, if appropriate, crosslinked, polyetheramides, polyvinylimidazoles, polyvinylpyrrolidines, polyvinylimidazolines, polyvinyltetrahydropyrines, poly(dialkylaminoalkyl vinylethers), poly(dialkylaminoalkyl(meth)acrylates) in protonated or in quaternized form and polyamidoamines obtained from a dicarboxylic acid, such as adipic acid, and polyalkylenepolyamines, such as diethylenetriamine, which are grafted with ethylenimine and crosslinked with polyethylene glycol dichlorohydrin ether according to the teaching of DE-B-24 34 816, or polyamidoamines which are reacted with epichlorohydrin to give water-soluble condensates. Further retention aids are cationic starches, alum and polyaluminum chloride.

Further suitable retention aids are so-called microparticle systems comprising a polymeric retention aid having a molar mass M_(w) of at least 1 million, preferably at least 2 million, and a finely divided inorganic or organic component. Such systems are known, cf. U.S. Pat. No. 3,052,595, EP-A-0 017 353, EP-A-0 223,223, EP-A-0 335 575, EP-A-0 711 371, WO-A-01/34910, U.S. Pat. No. 6,103,065 and DE-A-102 36 252. Both components are as a rule added separately from one another to the paper stock in the course of the papermaking process. Suitable organic components of the microparticle system are the polymers described above, for example retention aids from the group consisting of the polymers comprising vinylamine units, the polymers comprising vinylguanidine units, the nonionic, cationic and anionic polyacrylamides, polyethylenimines, crosslinked polyamidoamines grafted with ethylenimine, cationic starches and polydiallyldimethylammonium chlorides.

The polymeric retention aids of the microparticle system are added to the paper stock, for example, in an amount of from 0.005 to 0.5% by weight, preferably in an amount of from 0.01 to 0.25% by weight, based on dry paper stock.

Suitable inorganic components of the microparticle system are bentonite, colloidal silica, silicates and/or calcium carbonate. Colloidal silica is to be understood as meaning products which are based on silicates, e.g. silica microgel, silica sol, polysilicates, aluminum silicates, borosilicates, polyborosilicates, clay or zeolites. Calcium carbonate may be used, for example, in the form of chalk, ground calcium carbonate or precipitated calcium carbonate as an inorganic component of the microparticle system. Bentonite is understood as meaning generally sheet silicates which are swellable in water. These are in particular the clay mineral montmorillonite and similar clay minerals, such as nontronite, hectorite, saponite, sauconite, beidellite, allevardite, illite, halloysite, attapulgite and sepiolite. The sheet silicates are preferably activated before they are used, i.e. converted into a form swellable in water by treating the sheet silicates with an aqueous base, such as aqueous solutions of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, ammonia or amines. Bentonite in the form treated with sodium hydroxide solution is preferably used as an inorganic component of the microparticle system. The lamellae diameter of the bentonite dispersed in water is, for example, from 1 to 2 μm in the form treated with sodium hydroxide solution, and the thickness of the lamellae is about 1 nm. Depending on the type and activation, the bentonite has a specific surface area of from 60 to 800 m²/g. Typical bentonites are described, for example, in EP-B-0235893. In the papermaking process, bentonite is added to the cellulose suspension typically in the form of an aqueous bentonite slurry. This bentonite slurry may comprise up to 10% by weight of bentonite. Usually, the slurries comprise from about 3 to 5% by weight of bentonite.

Products from the group consisting of silicon-based particles, silica microgels, silica sols, aluminum silicates, borosilicates, polyborosilicates or zeolites can be used as colloidal silica. These have a specific surface area of from 50 to 1000 m²/g and an average particle size distribution of 1-250 nm, usually in the range of 40-100 nm. The preparation of such components is described, for example, in EP-A-0 041 056, EP-A-0 185 068 and U.S. Pat. No. 5,176,891.

Clay or kaolin is a water-containing aluminum silicate having a lamellar structure. The crystals have a layer structure and an aspect ratio (diameter-to-thickness ratio) of up to 30:1. The particle size is, for example, at least 50% smaller than 2 μm.

Preferably used carbonates are natural calcium carbonate (ground calcium carbonate, GCC) or precipitated calcium carbonate (PCC). GCC is prepared, for example, by milling and classifying processes with the use of milling assistants. It has a particle size of 40-95% smaller than 2 μm, and the specific surface area is in the range of 6-13 m²/g. PCC is prepared, for example, by passing carbon dioxide into an aqueous calcium hydroxide solution. The average particle size is in the range of 0.03-0.6 μm. The specific surface area may be strongly influenced by the choice of the precipitation conditions. It is in the range from 6 to 13 m²/g.

The inorganic component of the microparticle system is added to the paper stock in an amount of from 0.01 to 2.0% by weight, preferably in an amount of from 0.1 to 1.0% by weight, based on dry paper stock.

Combinations of an organic polymer having a molar mass M_(w) of at least 2 million and a mixture of a finely divided inorganic component and a finely divided organic component are also suitable as a microparticle system, the two components being metered independently of one another, either simultaneously or in succession. A suitable finely divided organic component having an anionic charge is described, for example, in WO-A-98/29604. At least one finely divided crosslinked copolymer of acrylamide and at least one monoethylenically unsaturated anionic monomer is preferably used as the finely divided, organic component of the microparticle system.

Examples of microparticle systems are combinations of cationic polymers, such as cationic starch, and finely divided silica or of cationic polymers, such as cationic polyacrylamide, and bentonite.

Instead of a retention aid or in combination with a retention aid, it is also possible to meter flocculants into at least one stream of a fiber suspension and/or into at least one stream of the water fed in. Those flocculants which are effective only at a relatively high temperature may be of particular interest here. An example of this is methyl cellulose, which is ineffective as a flocculant at about 20° C. and acts as a flocculent at 75° C., cf. G. V. Franks, Journal of Colloid and Interface Science 292, 598-603 (2005). A further example of a thermally sensitive flocculant is poly(N-isopropylacrylamide). The thermally sensitive flocculants can be metered in the process according to the invention, for example, into at least one stream of the water fed in, which has a temperature in the range in which the flocculant is effective. The stream of fibers may have, for example, a temperature in the range from 20 to 45° C. while the stream of the water fed in may have a temperature in the range from 60 to 75° C.

It is also possible to use pH-sensitive flocculants in the process according to the invention. An example of such a flocculant is the polysaccharide chitosan. It is ineffective as a flocculant, for example, at pH 4.5 but flocculates as soon as the pH is increased to, for example, 8.

Preferred drainage aids are polymers comprising ethylenimine units. Such polymers have already been mentioned above in the case of the cationic retention aids. They act both as retention aids and as drainage aids. Since the draining effect of this class of compounds is more pronounced than the retention effect, they are referred to in the present context as drainage aids. This class of compounds includes in particular polyethylenimines, which are obtainable by polymerization of ethylenimine in aqueous solution in the presence of acidic catalysts, such as mineral acids, or halogen compounds, such as methylene chloride, carbon tetrachloride, ethylene chloride or tetrachloroethane, and crosslinked ethylenimine-grafted condensates of a polyamidoamine and a dicarboxylic acid. Such products are sold under the trade mark Polymin® by BASF, Ludwigshafen. They are used in papermaking, for example, in an amount of at least 0.01% by weight, in general in the range from 0.1 to 0.3% by weight.

Depending on the fibers used in each case for the production of multilayer papers, a polyacrylamide and/or a polymer comprising vinylamine units are metered into at least one stream of a fiber suspension, and a polymer comprising ethylenimine units is metered into a stream of another fiber suspension.

In order to prepare, for example, a three-layer fiber web,

-   (a) a retention aid from the group consisting of polyacrylamides,     polymethacrylamides, polymers comprising vinylamine units,     microparticle systems and mixtures thereof is metered into the     stream of the fiber suspension which forms the top of the fiber web, -   (b) a drainage aid from the group consisting of the polymers     comprising ethylenimine units and/or a retention aid from the group     consisting of the polymers comprising vinylamine units, cationic,     anionic, nonionic and amphoteric polyacrylamides,     polymethacrylamides and mixtures thereof are metered into the stream     of the fiber suspension which forms the middle layer of the fiber     web, and -   (c) a retention aid from the group consisting of the     polyacrylamides, polymethacrylamides, polymers comprising vinylamine     units, microparticle systems and mixtures thereof is metered into     the stream of the fiber suspension which forms the bottom of the     fiber web.

In the production of a three-layer fiber web, preferably

-   (a) a polymer comprising vinylamine units is metered as a retention     aid into the stream of the fiber suspension which forms the top of     the fiber web, -   (b) a polymer comprising ethylenimine units is metered as a drainage     aid into the stream of the fiber suspension which forms the middle     layer of the fiber web, and -   (c) a polymer comprising vinylamine units is metered as a retention     aid into the stream of the fiber suspension which forms the bottom     of the fiber web.

Paper webs having improved formation are obtained, for example, when, in the production of a three-layer fiber web, a higher concentration of retention aid is used in the stream which forms the middle fiber web than in the streams which form the top and bottom of the fiber web. The amount of retention aid which is used for the middle layer of a three-layer fiber web is, for example, >0.01% by weight, in general from 0.015 to 0.3% by weight, based on dry paper stock. For the production of the top and the bottom of a three-layer fiber web, the amounts of retention aid are in general <0.01% by weight, for example in the range of from 0.001 to 0.009% by weight, based on dry fiber.

According to another process variant the addition of a retention aid or of a filler is effected with the aid of the water fed in. Thus, at least one retention aid and at least one thickener are metered into the stream of the water fed in. However, these additives can be mixed in the desired ratio in a storage container and transported therefrom into the nozzle chamber of the headbox. Suitable thickeners are, for example, high molecular weight polyacrylamides or high molecular weight polycarboxylic acids having molar masses M_(w) of at least 1 million, preferably at least three million. The thickeners are preferably crosslinked polyacrylamides or crosslinked polycarboxylic acids which swell very strongly in aqueous medium. An example of a known thickener is a crosslinked polyacrylic acid. Suitable crosslinking agents are, for example, methylenebisacrylamide, glycol diacrylate, butanediol diacrylate, butanediol dimethacrylate, pentaerythrityl triacrylate, trimethylolpropane triacrylate, pentaerythrityl triallylether or triallylamine The amounts of thickener added to the water are, for example, from 0.001 to 10% by weight, preferably from 0.01 to 1% by weight. An increase in the viscosity of the water is achieved thereby, with the result that the danger of mixing of the different fiber streams during the drainage process is reduced.

In a further process variant, the paper stock, preferably the low-consistency stock, and/or the stream of water fed in comprise at least one suspended filler. Suitable fillers are the finely divided inorganic substances usually used in papermaking, e.g. titanium dioxide, ground calcium carbonate (marble), precipitated calcium carbonate, chalk, talc, montmorillonite, dolomite or clay. Filler can be used, for example, in an amount of up to 40% by weight, in general in the range from 5 to 30% by weight, based in each case on dry paper stock. The fillers are used, for example, in the form of an aqueous pumpable slurry which comprises a dispersant, such as polyacrylic acid having a molar mass M_(w) of from 5000 to 12 000. The fillers can also be added by addition to the paper stock during the preparation of the pulp. The finely divided inorganic fillers generally lead to an increase in the basis weight of the filler-containing paper compared with a filler-free paper.

However, it is also possible to increase the volume of the paper by, for example, adding thermally expandable microparticles directly to the paper stock during the papermaking or, according to the invention, metering them as an aqueous suspension together with a retention aid and/or a drainage aid into the paper stock stream at a point which is after the last shearing stage of the paper stock and before the nozzle mouth of the headbox. In a further variant of the process according to the invention, an aqueous suspension of thermally expandable microparticles can also be metered into the stream of the water fed in, which counteracts mixing of the fiber streams. The microparticles are used, for example, in an amount of from 2 to 50% by weight, preferably from 5 to 45% by weight, based on dry paper stock.

Thermally expandable microparticles are known. They have, for example, a mean particle diameter from 17 to 35 μm. They are prepared by polymerization of ethylenically unsaturated monomers in the presence of a blowing agent and, if appropriate, further substances, such as silica, bentonite, clay, organic suspending media, such as methylcellulose, carboxymethylcellulose or polyvinyl alcohol, starch or oxides and hydroxides of aluminum, calcium, magnesium or barium. The blowing agent content of the microparticles is from 17 to 40% by weight, cf. US-A-2006/0102307. The microparticles described therein have a shell comprising a polymer of vinylidine chloride, acrylonitrile and methyl methacrylate. Further data on expandable microparticles can be found in the publications U.S. Pat. No. 3,615,972, U.S. Pat. No. 3,945,956, U.S. Pat. No. 5,536,756, U.S. Pat. No. 6,235,800, U.S. Pat. No. 6,235,394, U.S. Pat. No. 6,509,384 and EP-A-0 486,080. On drying of papers which comprise expandable microparticles, an increase in the volume of the paper occurs.

The effect of the drainage aids can be enhanced by using them together with at least one surface-active agent. Preferred surface-active agents are alkoxylation products of alcohols and amines. An example of this is Sursol® VL (BASF Aktiengesellschaft, Ludwigshafen). The amounts of surface-active agent are, for example, from 0.01 to 10% by weight, based on dry paper stock. Further agents of this category are quaternized alkanolamine-fatty acid esters, which are described, for example, in US-A-2006/0196624, or polyaminoamides, as described, for example, in Nordic Pulp and Paper Research Journal 2003, 18, 188-193.

In a further configuration of the process according to the invention, at least one retention aid is used in combination with an engine size. Preferred engine sizes are reactive size, in particular C₁₂- to C₂₂-alkylketene dimers, C₅- to C₂₂-alkyl- and/or C₅- to C₂₂-alkenylsuccinnic anhydrides, C₁₂- to C₃₆-alkyl isocyanates or mixtures of said compounds. It is also possible to use rosin size as an engine size. The aqueous reactive size dispersions are stabilized, for example, with the aid of cationic starch, cf. EP-B-0 353 212, EP-B-0 369 328 and EP-B-0 437 764. Both cationic and in particular anionic aqueous dispersions of at least one C₁₂- to C₂₂-alkyldiketene are used as sizes. Such dispersions are disclosed, for example, in WO-A-00/23651, pages 2 to 12. For the preparation of size dispersions, the reactive sizes are usually heated to a temperature which is above their melting point and then emulsified in water under the action of shear forces.

Liquid alkenylsuccinic anhydrides can be emulsified even at room temperature. For example, the customary homogenizers are used for emulsification. In order to stabilize the dispersed sizes in the aqueous phase, dispersants are used. Thus, for example, at least one ionic dispersant is used for the preparation of anionic size dispersions, for example a dispersant from the group consisting of the condensates of

(a) naphthalenesulfonic acid and formaldehyde, (b) phenol, phenolsulfonic acid and formaldehyde, (c) naphthalenesulfonic acid, formaldehyde and urea and (d) phenol, phenolsulfonic acid, formaldehyde and urea.

The anionic dispersant may be present in the form of the free acids, the alkali metal salts, alkaline earth metal salts and/or the ammonium salts. The ammonium salts may be derived both from ammonia and from primary, secondary and tertiary amines; for example, the ammonium salts of dimethylamine, trimethylamine, hexylamine cyclohexylamine, dicyclohexylamine, ethanolamine, diethanolamine and triethanolamine are suitable. The condensates described above are known and are commercially available. They are prepared by condensation of said constituents, it also being possible to use the corresponding alkali metal, alkaline earth metal or ammonium salts instead of the free acids. For example, acids, such as sulfuric acid, p-toluenesulfonic acid and phosphoric acid, are suitable as a catalyst in the condensation. Naphthalenesulfonic acid or the alkali metal salts thereof are condensed with formaldehyde, preferably in the molar ratio from 1:0.1 to 1:2 and in general in the molar ratio of from 1:0.5 to 1:1. The molar ratio for the preparation of condensates of phenol, phenolsulfonic acid and formaldehyde is likewise in the abovementioned range, any desired mixtures of phenol and phenolsulfonic acid being used instead of naphthalenesulfonic acid in the condensation with formaldehyde. Instead of phenolsulfonic acid, it is also possible to use the alkali metal and ammonium salts of phenolsulfonic acid. The condensation of the abovementioned starting material can, if appropriate, additionally be carried out in the presence of urea. For example, from 0.1 to 5 mol of urea, based on naphthalenesulfonic acid or on the mixture of phenol and phenolsulfonic acid, are used per mole of naphthalenesulfonic acid or per mole of the mixture of phenol and phenolsulfonic acid.

The condensates have, for example, molar masses in the range from 800 to 100 000, preferably from 1000 to 30 000 and in particular from 4000 to 25 000. Salts which are obtained, for example, on neutralization of the condensates with lithium hydroxide, sodium hydroxide, potassium hydroxide or ammonia are preferably used as anionic dispersants. The pH of the salts is, for example, in the range from 7 to 10.

Other suitable anionic dispersants are amphiphilic copolymers of

-   (i) hydrophobic monoethylenically unsaturated monomers and -   (ii) hydrophilic monomers having an anionic group, such as     monoethylenically unsaturated carboxylic acids, monoethylenically     unsaturated sulfonic acids, monoethylenically unsaturated phosphonic     acids or mixtures thereof.

Suitable hydrophobic monoethylenically unsaturated monomers (i) are, for example, olefins having 2 to 150 carbon atoms, styrene, α-methylstyrene, ethylstyrene, 4-methylstyrene, acrylonitrile, methacrylonitrile, esters of monoethylenically unsaturated C₃- to C₅-carboxylic acids and monohydric alcohols, amides of acrylic acid or methacrylic acid with C₁- to C₂₄-alkylamines, vinyl esters of saturated monocarboxylic acids having 2 to 24 carbon atoms, diesters of maleic acid or fumaric acid with monohydric C₁- to C₂₄-alcohols, vinyl ethers of alcohols having 3 to 24 carbon atoms or mixtures of said compounds.

The amphiphilic copolymers comprise, as hydrophilic monomers (ii), for example, monoethylenically unsaturated C₃- to C₁₀-carboxylic acids or anhydrides thereof, 2-acrylamido 2-methylpropanesulfonic acid, vinylsulfonic acid, styrenesulfonic acid, vinylphosphonic acid, salts of said monomers or mixtures thereof as hydrophilic monomers having an anionic group incorporated in the form of polymerized units.

Particularly preferred are aqueous size dispersions which comprise, as anionic dispersants, amphiphilic copolymers of

-   (i) α-olefins having 4 to 12 carbon atoms, styrene or mixtures     thereof as hydrophobic monomers and -   (ii) maleic acid, acrylic acid, methacrylic acid, monoesters of     maleic acid and alcohols having 1 to 25 carbon atoms or alkoxylation     products of such alcohols, monoamides of maleic acid, salts of said     monomers or mixtures of these compounds as hydrophilic monomers     having an anionic group     incorporated in the form polymerized units and have a molar mass     M_(w) of from 1500 to 100 000.

Preferably used anionic dispersants are copolymers of maleic anhydride with C₄- to C₁₂-olefins, particularly preferably C₈-olefins, such as 1-octene and diisobutene. Diisobutene is very particularly preferred. The molar ratio of maleic anhydride to olefin is, for example, in the range from 0.9:1 to 3:1, preferably from 0.95:1 to 1.5:1. These copolymers are preferably used in hydrolyzed form as aqueous solution or dispersions, the anhydride group being present in opened form and some or all of the carboxyl groups being neutralized. The following bases are used for the neutralization: alkali metal bases, such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, alkaline earth metal salts, such as calcium hydroxide, calcium carbonate, magnesium hydroxide, ammonia, primary, secondary or tertiary amines, such as triethylamine, triethanolamine, diethanolamine, ethanolamine, morpholine, etc.

If the amphiphilic copolymers are not sufficiently water-soluble in the form of the free acid, they are used in the form of water-soluble salts; for example, the corresponding alkali metal, alkaline earth metal and ammonium salts are used. The molar mass M_(w) of the amphiphilic copolymers is, for example, from 800 to 250 000, in general from 1000 to 100 000 and is preferably in the range from 3000 to 20 000, in particular from 1500 to 10 000. The acid numbers of the amphiphilic copolymers are, for example, from 50 to 500, preferably from 150 to 300 mg KOH/g of polymer.

The amphiphilic copolymers are used, for example, in amounts of from 0.05 to 20, preferably from 0.5 to 10, % by weight, based on the reactive size, as an anionic dispersant for the preparation of the size dispersions. The amphiphilic copolymers are preferably used in amounts of from 0.1 to 2, in particular from 0.6 to 1, % by weight, based on the size to be dispersed. With the use of amphiphilic copolymers alone as dispersants, aqueous size dispersions which are formaldehyde-free and have a long shelf life are obtained.

In order to prepare aqueous, anionic size dispersions, for example, an aqueous solution of at least one condensate or of at least one amphiphilic copolymer can be initially taken and the size dispersed therein at temperatures of, for example, from 20 to 100° C., preferably from 40 to 90° C. The size is preferably added in the form of a melt and is dispersed with vigorous stirring or shearing. The resulting dispersion is cooled in each case. In this way, it is possible to prepare, for example, aqueous, anionic size dispersions which comprise from 6 to 65% by weight of an alkyldiketene or from 0.1 to 65% by weight of alkylsuccinic anhydride as a size in dispersed form. Highly concentrated size dispersions which comprise, for example, from 25 to 60% by weight of alkyldiketene as a size in the presence of from 0.1 to 5.0% by weight of a condensate of naphthalenesulfonic acid and formaldehyde or of at least one condensate from (b), (c) and/or (d) in dispersed form are preferred.

Further preferred size dispersions comprise from 25 to 60% by weight of an alkyldiketene as a size and from 0.1 to 5.0% by weight of an amphiphilic copolymer of

-   (i) from 95 to 50% by weight of isobutene, diisobutene, styrene or     mixtures thereof and -   (ii) from 5 to 50% by weight of acrylic acid, methacrylic acid,     maleic acid, monoesters of maleic acid and/or mixtures thereof or a     water-soluble salt of such a copolymer.

Such highly concentrated size dispersions have a relatively low viscosity, for example in the range from 20 to 100 mPa·s (measured using a Brookfield viscometer and at a temperature of 20° C.) in the preparation of the aqueous dispersions, the pH is, for example, from 2 to 8 and preferably in the range from 3 to 4. Aqueous, anionic size dispersions having a mean particle size of the sizes in the range of from 0.1 to 3, preferably from 0.5 to 1.5, μm are obtained.

The anionically dispersed reactive sizes can, if appropriate, additionally comprise at least one cationic dispersant, but the amount of the cationic dispersant should be chosen so that the dispersion as a whole carries an anionic charge. A preferred cationic dispersant is cationic starch.

According to another process variant, the addition of a retention aid and/or of a filler is effected in combination with an engine size and/or a strength agent with the aid of the water fed in. Thus, at least one retention aid, a fixing agent and an engine size and/or a strength agent and, if appropriate, at least one thickener are metered, for example, into the stream of water fed in. However, these additives can be mixed in the desired ratio in a storage container and transported therefrom into the nozzle chamber of the headbox.

According to another process variant, the addition of a binder is effected alone or in combination with a filler, a retention aid, a fixing agent, an engine size and/or a strength agent, preferably with the aid of the water fed in, or it is metered into the paper stock.

Thus, at least one binder and optionally a filler, a retention aid, a fixing agent, an engine size, a strength agent and, if appropriate, at least one thickener are preferably metered into the stream of the water fed in. However, these additives can be mixed in the desired ratio in a storage container and transported therefrom into the nozzle chamber of the headbox. Binders produce, for example, better binding of fillers to the paper fibers and, if they are metered into the fiber streams which form the outer layers of the paper, improve the printability of the paper. With the aid of the binders, it is also possible to improve the barrier properties of paper, for example against the penetration of fats, oils and water and against the passage of gases, in particular oxygen or air.

Suitable synthetic binders are, for example, polymers which are composed of at least 40% by weight of so-called main monomers selected from C₁- to C₂₀-alkyl (meth)acrylates, vinyl esters of saturated carboxylic acids comprising up to 20 carbon atoms, vinylaromatics having up to 20 carbon atoms, ethylenically unsaturated nitriles, vinyl halides, vinyl ethers of alcohols comprising 1 to 10 carbon atoms, aliphatic hydrocarbons having 2 to 8 carbon atoms and one or two double bonds or mixtures of these monomers.

Particularly suitable synthetic polymers are polymers which are obtainable by free radical polymerization of ethylenically unsaturated compounds (monomers).

The binder is preferably a polymer which comprises at least 40% by weight, preferably at least 60% by weight, particularly preferably at least 80% by weight, of so-called main monomers.

The main monomers are selected from C₁-C₂₀-alkyl(meth)acrylates, vinyl esters of saturated carboxylic acids comprising up to 20 carbon atoms, vinylaromatics having up to 20 carbon atoms, ethylenically unsaturated nitriles, vinyl halides, vinyl ethers of alcohols comprising 1 to 10 carbon atoms, aliphatic hydrocarbons having 2 to 8 carbon atoms and one or two double bonds or mixtures of these monomers.

For example, alkyl(meth)acrylates having a C₁-C₁₀-alkyl radical, such as methyl methacrylate, methyl acrylate, n-butyl acrylate, ethyl acrylate and 2-ethylhexyl acrylate may be mentioned. Mixtures of the alkyl(meth)acrylates are also particularly suitable.

Vinyl esters of carboxylic acids having 1 to 20 carbon atoms are, for example, vinyl laurate, vinyl stearate, vinyl propionate, vinyl versatate and vinyl acetate.

Suitable vinylaromatic compounds are vinyl toluene, α- and p-methylstyrene, α-butylstyrene, 4-n-butylstyrene, 4-n-decylstyrene and preferably styrene. Examples of nitriles are acrylonitrile and methacrylonitrile.

The vinyl halides are ethylenically unsaturated compounds substituted by chlorine, fluorine or bromine, preferably vinyl chloride and vinylidene chloride.

For example, vinyl methyl ether or vinyl isobutyl ether may be mentioned as vinyl ethers. Vinyl ethers of alcohols comprising 1 to 4 carbon atoms are preferred.

Ethylene, propylene, butadiene, isoprene and chloroprene may be mentioned as hydrocarbons having 2 to 8 carbon atoms and one or two olefinic double bonds.

Preferred main monomers are C₁-C₁₀-alkyl(meth)acrylates and mixtures of the alkyl (meth)acrylates with vinylaromatics, in particular styrene (polymers having these main monomers are referred to for short as polyacrylates), or, alternatively, hydrocarbons having 2 double bonds, in particular butadiene, or mixtures of such hydrocarbons with vinylaromatics, in particular styrene (polymers having these main monomers are referred to for short as polybutadienes).

In the case of mixtures of aliphatic hydrocarbons (in particular butadiene) with vinylaromatics (in particular styrene), the ratio may be, for example, from 10:90 to 90:10, in particular from 20:80 to 80:20.

In addition to the main monomers, the polymer may comprise monomers having at least one acid group (acid monomer for short), for example monomers having carboxyl, sulfo or phosphonic acid groups. Carboxyl groups are preferred. For example, acrylic acid, methacrylic acid, itaconic acid, maleic acid or fumaric acid may be mentioned.

Further monomers are, for example, monomers comprising hydroxyl groups, in particular C₁-C₁₀-hydroxyalkyl(meth)acrylates, and (meth)acrylamide.

In the case of the polybutadienes, particularly preferred polymers are accordingly composed of

from 10 to 90% by weight, preferably from 20 to 70% by weight, of aliphatic hydrocarbons having two double bonds in particular butadiene, from 10 to 90% by weight, preferably from 30 to 80% by weight, of vinylaromatic compounds, in particular styrene, from 0 to 20% by weight, preferably from 0 to 10% by weight, of acid monomer and from 0 to 20% by weight, preferably from 0 to 10% by weight, of further monomers or alternatively, in the case of the polyacrylates, of from 10 to 95% by weight, preferably from 30 to 95% by weight, of C₁- to C₁₀-alkyl(meth)acrylates, from 0 to 60% by weight, preferably from 0 to 50% by weight, of vinylaromatic compounds, in particular styrene, from 0 to 20% by weight, preferably from 0 to 10% by weight, of acid monomer and from 0 to 20% by weight, preferably from 0 to 10% by weight, of further monomers.

Both the polybutadienes and the polyacrylates preferably comprise acid monomers as comonomers, preferably in an amount of from 1 to 5% by weight. The maximum amount of the above aliphatic hydrocarbons in the case of the polybutadienes or of the alkyl(meth)acrylates in the case of the polyacrylates decreases correspondingly by the minimum amount of the acid monomers.

In a preferred embodiment, the preparation of the polymers is effected by emulsion polymerization and the polymer is therefore an emulsion polymer. However, the polymers can, for example, also be prepared by solution polymerization and subsequent dispersing of the polymer solution in water.

In the case of the emulsion polymerization, ionic and/or nonionic emulsifiers and/or protective colloids or stabilizers are usually used as surface-active compounds.

The surface-active substance is used, for example, in amounts of from 0.1 to 10% by weight, based on the monomers to be polymerized.

Water-soluble initiators for the emulsion polymerization are, for example, ammonium and alkali metal salts of peroxodisulfuric acid, e.g. sodium peroxodisulfate, hydrogen peroxide or organic peroxides, e.g. tert-butyl hydroperoxide.

So-called reduction-oxidation (redox) initiator systems are also suitable.

The amount of the initiator is in general from 0.1 to 10% by weight, preferably from 0.5 to 5% by weight, based on the monomers to be polymerized. It is also possible to use a plurality of different initiators in the emulsion polymerization.

In the polymerization, it is possible to use regulators, for example in amounts of from 0 to 0.8 part by weight, based on 100 parts by weight of the monomers to be polymerized, by which the molar mass is reduced. For example, compounds having a thiol group, such as tert-butyl mercaptan, thioglycolic acid ethyl acrylic ester, mercaptoethanol, mercaptopropyltrimethoxysilane or tert-dodecyl mercaptan, are suitable.

The emulsion polymerization is effected as a rule at from 30 to 130° C., preferably from 50 to 90° C. The polymerization medium may consist either only of water or of mixtures of water and liquids miscible therewith, such as methanol. Preferably, only water is used. The emulsion polymerization can be carried out either as a batch process or in the form of a feed process, including the step or gradient procedure. The feed process in which a part of the polymerization batch is initially taken, heated to the polymerization temperature and prepolymerized and the remainder of the polymerization batch is then fed to the polymerization zone, usually via a plurality of spatially separated feeds, one or more of which comprise the monomers in pure or in emulsified form, continuously, stepwise or with superposition of a concentration gradient, while maintaining the polymerization, is preferred. A polymer seed may also be initially taken in the polymerization, for example for better adjustment of the particle size.

The manner in which the initiator is added to the polymerization vessel in the course of the free radical aqueous emulsion polymerization is known to the average person skilled in the art. It may be either completely initially taken in the polymerization vessel or used continuously or stepwise at the rate of its consumption in the course of the free radical aqueous emulsion polymerization. Specifically, this depends on the chemical nature of the initiator system as well as on the polymerization temperature. Preferably, a part is initially taken and the remainder is fed to the polymerization zone at the rate of consumption.

For removal of the residual monomers, initiator usually added even after the end of the actual emulsion polymerization, i.e. after the monomer conversion of at least 95%.

The individual components can be added to the reactor in the feed process from above, at the side or from below through the reactor bottom.

In the emulsion polymerization, aqueous dispersions of the polymer, as a rule having solids contents of from 15 to 75% by weight, preferably from 40 to 75% by weight, are obtained.

Particularly suitable binders are also mixtures of different binders, for example also mixtures of synthetic and natural polymers. Aqueous polymer dispersions which are composed of at least 60% by weight of butadiene or mixtures of butadiene and styrene or aqueous dispersions of polymers which comprise at least 60% by weight of C₁- to C₂₀-alkyl(meth)acrylates or mixtures of C₁- to C₂₀-alkyl(meth)acrylates with styrene incorporated in the form of polymerized units are preferably used as binders.

Other suitable binders are polymers which comprise N-vinylformamide and/or vinylamine units and have an average molar mass M, of at least 10 000. These polymers may be present as an aqueous dispersion or as a solution in water. They are prepared, for example, by polymerization of N-vinylformamide alone or in the presence of at least one other nonionic, cationic and/or anionic monomer. The homo- and copolymers of N-vinylformamide which can be prepared in this manner can be hydrolyzed in a polymer-analogous reaction with elimination of formyl groups from the vinylformamide units incorporated in the form of polymerized units, with formation of amino groups. The hydrolysis is preferably effected in an aqueous medium in the presence of at least one acid, such as hydrochloric acid or sulfuric acid, enzymatically or in the presence of bases, such as sodium hydroxide solution or potassium hydroxide solution. The vinylformamide units may be completely or only partly hydrolyzed. Thus, for example in the case of complete hydrolysis of homopolymers of N-vinylformamide, polyvinylamines are obtained.

Suitable anionic monomers are, for example, monomers comprising acid groups. Examples of these are acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, vinylphosphonic acid, acrylamido-2-methylpropanesulfonic acid, styrenesulfonic acid, allylacetic acid, crotonic acid and ethacrylic acid. The anionic monomers can be used in the polymerization in the form of the free acids or in a form partly or completely neutralized with alkali metal, alkaline earth metal and/or ammonium bases. The sodium salts or potassium salts of the acids are preferred. Both the unhydrolyzed copolymers of N-vinylformamide with anionic monomers and the partly or completely hydrolyzed copolymers of N-vinylformamide and anionic monomers, which are described, for example, in DE-A-103 15 363, cf. in particular page 5, line 39 to page 12, line 39, can be used as binders for modifying inorganic pigments.

N-vinylformamide can also be copolymerized with cationic monomers, such as dialkylaminoalkyl(meth)acrylates and/or diallyldimethylammonium chloride. The basic monomers are preferably used in the form of the salts with mineral acids or in a form partly or completely quaternized with alkyl halides or with dimethyl sulfate. In the case of the copolymerization of N-vinylformamide with anionic and/or cationic monomers, nonionic monomers, such as methyl acrylate, ethyl acrylate, methyl methacrylate, vinyl acetate, acrylamide and/or methacrylamide, can, if appropriate, additionally be used. Both the hydrolyzed cationic copolymers and the unhydrolyzed cationic copolymers can be used as binders for modifying the inorganic pigments. It is also possible to use amphoteric polymers which are obtainable, for example, by copolymerization of N-vinylformamide, dimethylaminoethyl acrylate methochloride and acrylic acid or which form as a result of complete or partial hydrolysis of the vinylformamide units of these copolymers. The polymers which comprise vinylformamide and/or vinylamine units and are used for modifying pigments preferably have an average molar mass M, of at least 20 000. In general, the average molar masses of the copolymers are in the range from 30 000 to 5 million, in particular from 50 000 to 2 million. The molar masses are determined, for example, with the aid of static light scattering at pH 7.6 in a 10 mmolar aqueous sodium chloride solution.

Also suitable as binders are ethylene copolymer waxes which comprise

-   (A′) from 20.5 to 38.9% by weight, preferably from 21 to 28% by     weight, of at least one ethylenically unsaturated carboxylic acid, -   (B′) from 60 to 79.4% by weight, preferably from 70 to 78.5% by     weight, of ethylene and -   (C′) from 0.1 to 15% by weight, preferably from 0.5 to 10% by     weight, of at least one ethylenically unsaturated carboxylate     incorporated in the form of polymerized units.

The ethylene copolymer waxes described above have, for example, a melt flow rate (MFR) in the range from 1 to 50 g/10 min, preferably from 5 to 20 g/10 min, particularly preferably from 7 to 15 g/10 min, measured at 160° C. and a load of 325 g according to EN ISO 1133. The acid number is usually from 100 to 300 mg KOH/g of wax, preferably from 115 to 230 mg KOH/g of wax, determined according to DIN 53402.

They have a kinematic melt viscosity n of at least 45 000 mm²/s, preferably of at least 50 000 mm²/s. The melting ranges of the ethylene copolymer waxes are, for example, in the range from 60 to 110° C., preferably in the range from 65 to 90° C., determined by DSC according to DIN 51007.

The melting ranges of the ethylene copolymer waxes may be broad and may have a temperature interval of at least 7 to not more than 20° C., preferably at least 10° C. and not more than 15° C.

The melting points of the ethylene copolymer waxes can, however, also have a small variation and may be in a temperature interval of less than 2° C., preferably less than 1° C., determined according to DIN 51007.

The density of the waxes is usually from 0.89 to 1.10 g/cm³, preferably from 0.92 to 0.99 g/cm³, determined according to DIN 53479.

The ethylene copolymer waxes are, for example, alternating copolymers or block copolymers or preferably random copolymers.

Ethylene copolymer waxes obtained from ethylene and ethylenically unsaturated carboxylic acids and, if appropriate, ethylenically unsaturated carboxylic esters can advantageously be prepared by free radical copolymerization under high pressure conditions, for example in high-pressure autoclaves which are equipped with a stirrer, or in high-pressure tubular reactors. The preparation in high-pressure autoclaves which are equipped with a stirrer is preferred. Such high-pressure autoclaves are known per se and a description is to be found in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, keyword: Waxes, vol. A28, page 146 et seq., Verlag Chemie, Weinheim, Basel, Cambridge, New York, Tokyo, 1996. In them, the length/diameter ratio is predominantly in the ranges from 5:1 to 30:1, preferably from 10:1 to 20:1. The high-pressure tubular reactors which can likewise be used are also described in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, keyword: Waxes, vol. A28, page 146 et seq., Verlag Chemie, Weinheim, Basel, Cambridge, New York, Tokyo, 1996.

Suitable pressure conditions for the polymerization are from 500 to 4000 bar, preferably from 1500 to 2500 bar. Conditions of this type are also referred to below as high-pressure. The reaction temperatures are in the range from 170 to 300° C., preferably in the range from 195 to 280° C. The polymerization can also be carried out in the presence of a regulator. The abovementioned waxes are described in detail, for example, in WO-A-04/108601, page 2, line 38 to page 12, line 10.

According to another variant of the process according to the invention, the addition of an unstabilized engine size alone or in combination with a filler, a retention aid, a fixing agent and/or a strength agent is effected with the aid of the water fed in. Thus, at lest one unstabilized engine size and optionally a filler, a retention aid, a fixing agent, a strength agent and at least one thickener are metered into the stream of the water fed in. However, these additives can be mixed in the desired ratio in the storage container and transported therefrom into the nozzle chamber of the headbox.

Unstabilized engine sizes are to be understood as meaning the above-described engine sizes which contain no stabilizers or very small proportions of stabilizers. If the engine size dispersion is metered into the stream of the water fed in, directly after emulsification according to the above-described variant of the process according to the invention, the engine sizes can be emulsified on site.

The aqueous dispersions of an engine size can, if appropriate, be used together with a cationic, synthetic polymer acting as a fixing agent and promoter, fixing agent and promoter being metered as a mixture with at least one engine size or separately therefrom into the paper stock. The metering is generally effected into the low-viscosity stock. For optimum metering, engine size and, if appropriate, promoter are metered in combination with a retention aid into the paper stock stream at a point which is after the last shearing stage of the paper stock and before the nozzle mouth of the headbox. The cationic polymers used as fixing agent and promoter for engine sizes can be metered into the paper stock, for example, before or after the last shearing stage. Examples of cationic polymers of this type are polymers comprising vinylamine units, polymers comprising vinylguanidine units, polyethylenimines, polyamidoamines grafted with ethylenimine and/or polydiallyldimethylammonium chlorides. The amount of fixing agents is, for example, from 0.02 to 2.0, preferably from 0.05 to 0.5, % by weight, based on dry paper stock.

In another embodiment of the invention, at least one retention aid is used in combination with a strength agent for paper. Known strength agents for paper are, for example, urea-formaldehyde resins which increase not only the wet strength but also the dry strength of the paper (cf. EP-A-0 123 196 and U.S. Pat. No. 3,275,605), melamine-formaldehyde resins (cf. DE-B-10 90 078) or other commercially available products, for example polyamidoamines crosslinked with epichlorohydrin (cf. the Luresin® brands, BASF Aktiengesellschaft, Ludwigshafen).

In order to be able to prepare multilayer webs from cellulose fibers, it is possible to start from all fibers customary in papermaking. The cellulose fibers are first suspended in water for the preparation of a paper stock. Suitable cellulose fibers are, for example, fibers obtained from mechanical pulp and all annual plants. Mechanical pulp includes, for example, groundwood, thermomechanical pulp (TMP), chemothermomechanical pulp (CTMP), pressure groundwood, semi-chemical pulp, high-yield pulp and refiner mechanical pulp (RMP) and wastepaper. Chemical pulps which can be used in bleached or in unbleached form, such as sulfate, sulfite and soda pulps, and fibers obtained from wastepaper are also suitable. Unbleached chemical pulps, which are also referred to as unbleached kraft pulp, are preferably used. Said fibers can be used alone or as a mixture. The use of kraft pulp and of TMP and CTMP is particularly preferred. The pH of the cellulose fiber slurry is, for example, from 4 to 8, preferably from 6 to 8. The consistency of the pulp which is drained in the paper machine is not more than 2% by weight and is in general in the range from 0.5 to 1.0% by weight, based on dry paper stock.

The process chemicals usually used in papermaking can also be added in the usual amounts to the paper stock, for example the abovementioned fixing agents, sizes, dry and wet strength agents, biocides and/or dyes. The paper stock is processed in each case according to the invention to give a multilayer fiber web by draining it on a wire with web formation. The webs thus produced are dried. Draining of the paper stock and drying of the webs are part of the papermaking process, which is carried out continuously.

In the process according to the invention, for example, it is possible to produce papers which are composed of 2, 3, 4, 5 or more layers. Three-layer papers are preferred. It is in fact possible here to produce good-quality papers economically by using cheap fibers for the middle layer and fibers of better quality for the top and bottom of the three-layer paper. Thus, bleached fibers, such as bleached fibers of birch sulfate and/or pine sulfate can be used, for example, for the formation of the top and bottom of a three-layer paper while fibers from wastepaper, TMP and/or groundwood are suitable for the middle layer of the three-layer paper.

The multilayer papers produced according to the invention are suitable, for example, as printing and writing papers, copying papers, inkjet papers, cardboard and board and for the packaging of liquids and the production of folding boxes and corrugated board and cardboard. 

1. A process for the production of a multilayer fiber web from cellulose fibers by separately feeding in each case at least two different fiber suspensions and water into a multilayer headbox, in which they are separated by separating elements from one another and from water fed in and, after leaving the nozzle mouth of the headbox, reach a drainage apparatus on which a multilayer fiber web is formed, the water transported in such a way that, after emerging from the nozzle mouth, it reaches the web-forming apparatus in the form of a layer between two layers of fiber suspensions and thus counteracts mixing of the different fiber suspensions, wherein at least one retention aid, at least one drainage aid, or a combination thereof is metered into at least one of the fiber suspensions the water fed in.
 2. The process according to claim 1, wherein an aqueous solution of at least one retention aid, at least one drainage aid, or a combination thereof is metered into the paper stock stream at a point which is located after the last shearing stage of the paper stock and before the nozzle mouth of the headbox.
 3. The process according to claim 1, wherein the same retention aid is added to all streams of fiber suspensions and it is metered in such a way that at least two streams of fiber suspensions comprise a different concentration of this retention aid.
 4. The process according to claim 1, wherein at least two retention aids differing from one another are metered into at least two streams of fiber suspensions.
 5. The process according to claim 4, wherein at least two streams of fiber suspensions comprise a different concentration of retention aid.
 6. The process according to claim 1, wherein an aqueous solution of a retention aid and an aqueous dispersion of at least one filler are metered separately from one another or as a mixture into the paper stock.
 7. The process according to claim 1, wherein an aqueous solution of a retention aid and an aqueous dispersion of at least one filler are metered separately from one another or as a mixture into at least one stream of the water fed in.
 8. The process according to claim 1, wherein an aqueous dispersion of a filler is metered into at least one stream of the fiber suspension and wherein the water fed in comprises at least one retention aid.
 9. The process according to claim 1, wherein at least one retention aid is present in combination with at least one drainage aid.
 10. The process according to claim 1, wherein at least one retention aid is present in combination with a fixing agent.
 11. The process according to claim 1, wherein at least one retention aid is present in combination with an engine size.
 12. The process according to claim 1, wherein at least one retention aid is present in combination with a paper strength agent.
 13. The process according to claim 1, wherein the retention aid is at least one member selected from the group consisting of a cationic polymeric organic compound, an anionic polymeric organic compound, a nonionic polymeric organic compound an amphoteric polymeric organic compound, and a microparticle system.
 14. The process according to claim 1, wherein the retention aid is at least one member selected from the group consisting of a polyacrylamide, a polymethacrylamide a polymer comprising vinylamine units and a microparticle system.
 15. The process according to claim 1, wherein at least one retention aid is a polymer comprising vinylamine units.
 16. The process according to claim 1, wherein the drainage aid is a polymer comprising ethylenimine units.
 17. The process according to claim 1, wherein at least one of a polyacrylamide and a polymer comprising vinylamine units are metered into at least one stream of a fiber suspension, and a polymer comprising ethylenimine units is metered into a stream of another fiber suspension.
 18. The process according to claim 1, wherein a three-layer fiber web is produced, (a) a retention aid selected from the group consisting of a polyacrylamide, a polymethacrylamide, a polymer comprising vinylamine units, a microparticle system, and a mixture thereof is metered into the stream of the fiber suspension which forms the top of the fiber web, (b) a drainage aid of a polymer comprising ethylenimine units, a retention aid selected from the group consisting of a polymer comprising vinylamine units, a cationic polyacrylamide, an anionic polyacrylamide, a nonionic polyacrylamide, a amphoteric polyacrylamide, a polymethacrylamide, and mixtures thereof, or a combination thereof is metered into the stream of the fiber suspension which forms the middle layer of the fiber web, and (c) a retention aid selected from the group consisting of a polyacrylamide, a polymethacrylamide, a polymer comprising vinylamine units, a microparticle system, and a mixture thereof is metered into the stream of the fiber suspension which forms the bottom of the fiber web.
 19. The process according to claim 1, wherein a three-layer fiber web is produced, (a) a polymer comprising vinylamine units is metered as a retention aid into the stream of the fiber suspension which forms the top of the fiber web, (b) a polymer comprising ethylenimine units is metered as a drainage aid into the stream of the fiber suspension which forms the middle layer of the fiber web, and (c) a polymer comprising vinylamine units is metered as a retention aid into the stream of the fiber suspension which forms the bottom of the fiber web.
 20. The process according to claim 1, wherein the stream of water fed in comprises at least one retention aid and at least one thickener.
 21. The process according to claim 1, wherein the stream of water fed in comprises at least one retention aid and at least one binder.
 22. The process according to claim 1, wherein the stream of water fed in comprises at least one suspended filler.
 23. The process according to claim 1, wherein the stream of water fed in comprises at least one retention aid, a fixing agent, an engine size, a strength agent, a thickener or a combination thereof.
 24. The process according to claim 1, wherein an unstabilized engine size is metered into the stream of water fed in.
 25. The process according to claim 1, wherein a surface-active agent is present for improving the drainage. 